High-speed light sensing apparatus

ABSTRACT

An apparatus including a semiconductor substrate; an absorption layer coupled to the semiconductor substrate, the absorption layer including a photodiode region configured to absorb photons and to generate photo-carriers from the absorbed photons; one or more first switches controlled by a first control signal, the one or more first switches configured to collect at least a portion of the photo-carriers based on the first control signal; and one or more second switches controlled by a second control signal, the one or more second switches configured to collect at least a portion of the photo-carriers based on the second control signal, where the second control signal is different from the first control signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation application of and claimspriority to U.S. application Ser. No. 17/344,301, filed on Jun. 10,2021, which a continuation application of and claims priority to U.S.application Ser. No. 16/855,163, filed on Apr. 22, 2020, now U.S. Pat.No. 11,131,757, which is a continuation application of and claimspriority to U.S. application Ser. No. 16/430,019, filed on Jun. 3, 2019,now U.S. Pat. No. 10,795,003, which is a continuation application of andclaims priority to U.S. application Ser. No. 16/146,656, filed on Sep.28, 2018, now U.S. Pat. No. 10,353,056, which is a continuationapplication of and claims priority to U.S. application Ser. No.15/338,660, filed on Oct. 31, 2016, now U.S. Pat. No. 10,254,389, whichclaims the benefit of U.S. Provisional Patent Application No.62/251,691, filed Nov. 6, 2015, U.S. Provisional Patent Application No.62/271,386, filed Dec. 28, 2015, U.S. Provisional Patent Application No.62/294,436, filed Feb. 12, 2016, all of which are incorporated byreference herein.

BACKGROUND

This specification relates to detecting light using a photodiode.

Light propagates in free space or an optical medium is coupled to aphotodiode that converts an optical signal to an electrical signal forprocessing.

SUMMARY

According to one innovative aspect of the subject matter described inthis specification, light reflected from a three-dimensional object maybe detected by photodiodes of an imaging system. The photodiodes convertthe detected light into electrical charges. Each photodiode may includetwo groups of switches that collect the electrical charges. Thecollection of the electrical charges by the two groups of switches maybe altered over time, such that the imaging system may determine phaseinformation of the sensed light. The imaging system may use the phaseinformation to analyze characteristics associated with thethree-dimensional object including depth information or a materialcomposition. The imaging system may also use the phase information toanalyze characteristics associated with eye-tracking, gesturerecognition, 3-dimensional model scanning/video recording, and/oraugmented/virtual reality applications.

In general, one innovative aspect of the subject matter described inthis specification can be embodied in an optical apparatus that includesa semiconductor substrate; a germanium-silicon layer coupled to thesemiconductor substrate, the germanium-silicon layer including aphotodiode region configured to absorb photons and to generatephoto-carriers from the absorbed photons; one or more first switchescontrolled by a first control signal, the one or more first switchesconfigured to collect at least a portion of the photo-carriers based onthe first control signal; and one or more second switches controlled bya second control signal, the one or more second switches configured tocollect at least a portion of the photo-carriers based on the secondcontrol signal, where the second control signal is different from thefirst control signal. The one or more first switches include a firstp-doped region in the germanium-silicon layer, where the first p-dopedregion is controlled by the first control signal; and a first n-dopedregion in the germanium-silicon layer, where the first n-doped region iscoupled to a first readout integrated circuit. The one or more secondswitches include a second p-doped region in the germanium-silicon layer,where the second p-doped region is controlled by the second controlsignal; and a second n-doped region in the germanium-silicon layer,where the second n-doped region is coupled to a second readoutintegrated circuit.

This and other implementations can each optionally include one or moreof the following features. The germanium-silicon layer may include athird n-doped region and a fourth n-doped region, where at least aportion of the first p-doped region may be formed in the third n-dopedregion, and where at least a portion of the second p-doped region may beformed in the fourth n-doped region. The germanium-silicon layer mayinclude a third n-doped region, where at least a portion of the firstp-doped region and a portion of the second p-doped region may be formedin the third n-doped region. The semiconductor substrate may include athird p-doped region and one or more n-doped regions, where thegermanium-silicon layer may be arranged over the third p-doped region,and where the third p-doped region may be electrically shorted with theone or more n-doped regions.

The first control signal may be a fixed bias voltage, and the secondcontrol signal may be a variable bias voltage that is biased over thefixed voltage of the first control signal. The photons absorbed by thegermanium-silicon layer may be reflected from a surface of athree-dimensional target, and the portion of the photo-carrierscollected by the one or more first switches and the portion of thephoto-carriers collected by the one or more second switches may beutilized by a time-of-flight system to analyze depth information or amaterial composition of the three-dimensional target.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus including asemiconductor substrate; an absorption layer coupled to thesemiconductor substrate, the absorption layer including a photodioderegion configured to absorb photons and to generate photo-carriers fromthe absorbed photons; one or more first switches controlled by a firstcontrol signal, the one or more first switches configured to collect atleast a portion of the photo-carriers based on the first control signal;and one or more second switches controlled by a second control signal,the one or more second switches configured to collect at least a portionof the photo-carriers based on the second control signal, where thesecond control signal is different from the first control signal. Theone or more first switches include a first p-doped region in thesemiconductor substrate, where the first p-doped region is controlled bythe first control signal; and a first n-doped region in thesemiconductor substrate, where the first n-doped region is coupled to afirst readout integrated circuit. The one or more second switchesinclude a second p-doped region in the semiconductor substrate, wherethe second p-doped region is controlled by the second control signal;and a second n-doped region in the semiconductor substrate, wherein thesecond n-doped region is coupled to a second readout integrated circuit.

This and other implementations can each optionally include one or moreof the following features. The semiconductor substrate may include athird n-doped region and a fourth n-doped region, where at least aportion of the first p-doped region may be formed in the third n-dopedregion, and where at least a portion of the second p-doped region may beformed in the fourth n-doped region. The semiconductor substrate mayinclude a third n-doped region, where at least a portion of the firstp-doped region and a portion of the second p-doped region may be formedin the third n-doped region. The semiconductor substrate may include oneor more p-well regions.

The first control signal may be a fixed bias voltage, where the secondcontrol signal may be a variable bias voltage that is biased over thefixed voltage of the first control signal. The photons absorbed by theabsorption layer may be reflected from a surface of a three-dimensionaltarget, where the portion of the photo-carriers collected by the one ormore first switches and the portion of the photo-carriers collected bythe one or more second switches may be utilized by a time-of-flightsystem to analyze depth information or a material composition of thethree-dimensional target.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus including asemiconductor substrate; an absorption layer coupled to thesemiconductor substrate, the absorption layer including a photodioderegion configured to absorb photons and to generate photo-carriers fromthe absorbed photons; one or more first switches controlled by a firstcontrol signal, the one or more first switches configured to collect atleast a portion of the photo-carriers based on the first control signal;and one or more second switches controlled by a second control signal,the one or more second switches configured to collect at least a portionof the photo-carriers based on the second control signal, where thesecond control signal is different from the first control signal. Theone or more first switches include multiple first p-doped regions in thesemiconductor substrate, where the multiple first p-doped regions arecontrolled by the first control signal; and multiple first n-dopedregions in the semiconductor substrate, where the multiple first n-dopedregions are coupled to a first readout integrated circuit. The one ormore second switches include multiple second p-doped regions in thesemiconductor substrate, where the multiple second p-doped regions arecontrolled by the second control signal; and multiple second n-dopedregions in the semiconductor substrate, where the multiple secondn-doped regions are coupled to a second readout integrated circuit.

This and other implementations can each optionally include one or moreof the following features. The semiconductor substrate may include athird n-doped region, where at least a portion of the multiple firstp-doped region and a portion of the multiple second p-doped region maybe formed in the third n-doped region. The multiple first p-dopedregions and the multiple second p-doped regions may be arranged in aninterdigitated arrangement along a first plane in the semiconductorsubstrate, where the multiple first n-doped regions and the multiplesecond n-doped regions may be arranged in an interdigitated arrangementalong a second plane in the semiconductor substrate that is differentfrom the first plane. Each p-doped region of the multiple first p-dopedregions may be arranged over a respective n-doped region of the multiplefirst n-doped regions, and each p-doped region of the multiple secondp-doped regions may be arranged over a respective n-doped region of themultiple second n-doped regions. The semiconductor substrate may includeone or more p-well regions.

The first control signal may be a fixed bias voltage, and the secondcontrol signal may be a variable bias voltage that is biased over thefixed voltage of the first control signal. The photons absorbed by theabsorption layer may be reflected from a surface of a three-dimensionaltarget, where the portion of the photo-carriers collected by the one ormore first switches and the portion of the photo-carriers collected bythe one or more second switches may be utilized by a time-of-flightsystem to analyze depth information or a material composition of thethree-dimensional target.

Another innovative aspect of the subject matter described in thisspecification can be embodied in a time-of-flight system that includes alight source; and an image sensor comprising multiple pixels fabricatedon a semiconductor substrate, where each pixel of the pixels includes agermanium-silicon layer coupled to the semiconductor substrate, thegermanium-silicon layer including a photodiode region configured toabsorb photons and to generate photo-carriers from the absorbed photons;one or more first switches controlled by a first control signal, the oneor more first switches configured to collect at least a portion of thephoto-carriers based on the first control signal; and one or more secondswitches controlled by a second control signal, the one or more secondswitches configured to collect at least a portion of the photo-carriersbased on the second control signal, where the second control signal isdifferent from the first control signal.

This and other implementations can each optionally include one or moreof the following features. The light source may be configured to emitoptical pulses having a duty cycle that is less than 50% but maintaininga same amount of energy per optical pulse.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus that includes asemiconductor substrate; a germanium-silicon layer coupled to thesemiconductor substrate, the germanium-silicon layer including aphotodiode region configured to absorb photons and to generatephoto-carriers from the absorbed photons; one or more first switchescontrolled by a first control signal, the one or more first switchesconfigured to collect at least a portion of the photo-carriers based onthe first control signal; and one or more second switches controlled bya second control signal, the one or more second switches configured tocollect at least a portion of the photo-carriers based on the secondcontrol signal, where the second control signal is different from thefirst control signal. The one or more first switches include a firstp-doped region in the germanium-silicon layer, where the first p-dopedregion is controlled by the first control signal; and a first n-dopedregion in the semiconductor substrate, where the first n-doped region iscoupled to a first readout integrated circuit. The one or more secondswitches include a second p-doped region in the germanium-silicon layer,where the second p-doped region is controlled by the second controlsignal; and a second n-doped region in the semiconductor substrate,where the second n-doped region is coupled to a second readoutintegrated circuit.

This and other implementations can each optionally include one or moreof the following features. The germanium-silicon layer may include athird n-doped region and a fourth n-doped region, where at least aportion of the first p-doped region is formed in the third n-dopedregion, and where at least a portion of the second p-doped region may beformed in the fourth n-doped region. The germanium-silicon layer mayinclude a third n-doped region, where at least a portion of the firstp-doped region and a portion of the second p-doped region may be formedin the third n-doped region. The semiconductor substrate may include oneor more p-well regions.

The first control signal may be a fixed bias voltage, where the secondcontrol signal may be a variable bias voltage that is biased over thefixed voltage of the first control signal. The photons absorbed by thegermanium-silicon layer may be reflected from a surface of athree-dimensional target, where the portion of the photo-carrierscollected by the one or more first switches and the portion of thephoto-carriers collected by the one or more second switches may beutilized by a time-of-flight system to analyze depth information or amaterial composition of the three-dimensional target.

Advantageous implementations may include one or more of the followingfeatures. Germanium is an efficient absorption material fornear-infrared wavelengths, which reduces the problem of slowphoto-carriers generated at a greater substrate depth when aninefficient absorption material, e.g., silicon, is used. For aphotodiode having p- and n-doped regions fabricated at two differentdepths, the photo-carrier transit distance is limited by the depth, andnot the width, of the absorption material. Consequently, if an efficientabsorption material with a short absorption length is used, the distancebetween the p- and n-doped regions can also be made short so that even asmall bias may create a strong field resulting into an increasedoperation speed. For such a photodiode, two groups of switches may beinserted and arranged laterally in an interdigitated arrangement, whichmay collect the photo-carriers at different optical phases for atime-of-flight system. An increased operation speed allows the use of ahigher modulation frequency in a time-of-flight system, giving a greaterdepth resolution. In a time-of-flight system where the peak intensity ofoptical pulses is increased while the duty cycle of the optical pulsesis decreased, the signal-to-noise ratio can be improved whilemaintaining the same power consumption for the time-of-flight system.This is made possible when the operation speed is increased so that theduty cycle of the optical pulses can be decreased without distorting thepulse shape.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other potentialfeatures and advantages will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, and 1D are examples of a dual-switch photodiode.

FIGS. 2A, 2B, 2C and 2D are examples of a dual-switch photodiode.

FIGS. 3A, 3B, 3C, and 3D are examples of a dual-switch photodiode.

FIGS. 4A, 4B, 4C, 4D, and 4E are examples of a dual-switch photodiode.

FIG. 5A is a block diagram of an example of an imaging system.

FIGS. 5B and 5C show examples of techniques for determiningcharacteristics of an object using an imaging system.

FIG. 6 shows an example of a flow diagram for determiningcharacteristics of an object using an imaging system.

Like reference numbers and designations in the various drawings indicatelike elements. It is also to be understood that the various exemplaryembodiments shown in the figures are merely illustrative representationsand are not necessarily drawn to scale.

DETAILED DESCRIPTION

Photodiodes may be used to detect optical signals and convert theoptical signals to electrical signals that may be further processed byanother circuitry. In time-of-flight (TOF) applications, depthinformation of a three-dimensional object may be determined using aphase difference between a transmitted light pulse and a detected lightpulse. For example, a two-dimensional array of pixels may be used toreconstruct a three-dimensional image of a three-dimensional object,where each pixel may include one or more photodiodes for deriving phaseinformation of the three-dimensional object. In some implementations,time-of-flight applications use light sources having wavelengths in thenear-infrared (NIR) range. For example, a light-emitting-diode (LED) mayhave a wavelength of 850 nm, 940 nm or 1050 nm. Some photodiodes may usesilicon as an absorption material, but silicon is an inefficientabsorption material for NIR wavelengths. Specifically, photo-carriersmay be generated deeply (e.g., greater than 10 μm in depth) in thesilicon substrate, and those photo-carriers may drift and/or diffuse tothe photodiode junction slowly, which results in a decrease in theoperation speed. Moreover, a small voltage swing is typically used tocontrol photodiode operations in order to minimize power consumption.For a large absorption area (e.g., 10 μm in diameter), the small voltageswing can only create a small lateral/vertical field across the largeabsorption area, which affects the drift velocity of the photo-carriersbeing swept across the absorption area. The operation speed is thereforefurther limited. For TOF applications using NIR wavelengths, adual-switch photodiode with innovative design structures and/or with theuse of germanium-silicon (GeSi) as an absorption material addresses thetechnical issues discussed above. In this application, the term“photodiode” may be used interchangeably with the term “optical sensor”.In this application, the term “germanium-silicon (GeSi)” refers to aGeSi alloy with alloy composition ranging from more than 10% germanium(Ge), i.e., less than 90% silicon (Si), to 100% Ge, i.e., 0% of Si. Inthis application, the GeSi material may be grown using a blanketepitaxy, a selective epitaxy, or other applicable techniques.Furthermore, an absorption layer comprising the GeSi material may beformed on a planar surface, a mesa top surface, or a trench bottomsurface at least partially surrounded by an insulator (ex: oxide,nitrite), a semiconductor (ex: Si, Ge), or their combinations.Furthermore, a strained super lattice structure or a multiple quantumwell structure including alternative layers such as GeSi layers with twoor more different alloy compositions may be used for the absorptionlayer.

FIG. 1A is an example dual-switch photodiode 100 for converting anoptical signal to an electrical signal. The dual-switch photodiode 100includes an absorption layer 106 fabricated on a substrate 102. Thesubstrate 102 may be any suitable substrate where semiconductor devicescan be fabricated on. For example, the substrate 102 may be a siliconsubstrate. The absorption layer 106 includes a first switch 108 and asecond switch 110.

In general, the absorption layer 106 receives an optical signal 112 andconverts the optical signal 112 into electrical signals. The absorptionlayer 106 is selected to have a high absorption coefficient at thedesired wavelength range. For NIR wavelengths, the absorption layer 106may be a GeSi mesa, where the GeSi absorbs photons in the optical signal112 and generates electron-hole pairs. The material composition ofgermanium and silicon in the GeSi mesa may be selected for specificprocesses or applications. In some implementations, the absorption layer106 is designed to have a thickness t. For example, for 850 nmwavelength, the thickness of the GeSi mesa may be approximately 1 μm tohave a substantial quantum efficiency. In some implementations, thesurface of the absorption layer 106 is designed to have a specificshape. For example, the GeSi mesa may be circular, square, orrectangular depending on the spatial profile of the optical signal 112on the surface of the GeSi mesa. In some implementations, the absorptionlayer 106 is designed to have a lateral dimension d for receiving theoptical signal 112. For example, the GeSi mesa may have a circularshape, where d can range from 1 μm to 50 μm.

A first switch 108 and a second switch 110 have been fabricated in theabsorption layer 106. The first switch 108 is coupled to a first controlsignal 122 and a first readout circuit 124. The second switch 110 iscoupled to a second control signal 132 and a second readout circuit 134.In general, the first control signal 122 and the second control signal132 control whether the electrons or the holes generated by the absorbedphotons are collected by the first readout circuit 124 or the secondreadout circuit 134.

In some implementations, the first switch 108 and the second switch 110may be fabricated to collect electrons. In this case, the first switch108 includes a p-doped region 128 and an n-doped region 126. Forexample, the p-doped region 128 may have a p+ doping, where theactivated dopant concentration may be as high as a fabrication processmay achieve, e.g., the peak concentration may be about 5×10²⁰ cm⁻³ whenthe absorption layer 106 is germanium and doped with boron. In someimplementation, the doping concentration of the p-doped region 128 maybe lower than 5×10²⁰ cm⁻³ to ease the fabrication complexity at theexpense of an increased contact resistance. The n-doped region 126 mayhave an n+ doping, where the activated dopant concentration may be ashigh as a fabrication process may achieve, e.g., the peak concentrationmay be about 1×10²⁰ cm⁻³ when the absorption layer 106 is germanium anddoped with phosphorous. In some implementation, the doping concentrationof the n-doped region 126 may be lower than 1×10²⁰ cm⁻³ to ease thefabrication complexity at the expense of an increased contactresistance. The distance between the p-doped region 128 and the n-dopedregion 126 may be designed based on fabrication process design rules. Ingeneral, the closer the distance between the p-doped region 128 and then-doped region 126, the higher the switching efficiency of the generatedphoto-carriers. The second switch 110 includes a p-doped region 138 andan n-doped region 136. The p-doped region 138 is similar to the p-dopedregion 128, and the n-doped region 136 is similar to the n-doped region126.

In some implementations, the p-doped region 128 is coupled to the firstcontrol signal 122. For example, the p-doped region 128 may be coupledto a voltage source, where the first control signal 122 may be an ACvoltage signal from the voltage source. In some implementations, then-doped region 126 is coupled to the readout circuit 124. The readoutcircuit 124 may be in a three-transistor configuration consisting of areset gate, a source-follower, and a selection gate, or any suitablecircuitry for processing charges. In some implementations, the readoutcircuit 124 may be fabricated on the substrate 102. In some otherimplementations, the readout circuit 124 may be fabricated on anothersubstrate and integrated/co-packaged with the dual-switch photodiode 100via die/wafer bonding or stacking.

The p-doped region 138 is coupled to the second control signal 132. Forexample, the p-doped region 138 may be coupled to a voltage source,where the second control signal 132 may be an AC voltage signal havingan opposite phase from the first control signal 122. In someimplementations, the n-doped region 136 is coupled to the readoutcircuit 134. The readout circuit 134 may be similar to the readoutcircuit 124.

The first control signal 122 and the second control signal 132 are usedto control the collection of electrons generated by the absorbedphotons. For example, when voltages are used, if the first controlsignal 122 is biased against the second control signal 132, an electricfield is created between the p-doped region 128 and the p-doped region138, and free electrons drift towards the p-doped region 128 or thep-doped region 138 depending on the direction of the electric field. Insome implementations, the first control signal 122 may be fixed at avoltage value V_(i), and the second control signal 132 may alternatebetween voltage values V_(i)±ΔV. The direction of the bias valuedetermines the drift direction of the electrons. Accordingly, when oneswitch (e.g., the first switch 108) is switched “on” (i.e., theelectrons drift towards the p-doped region 128), the other switch (e.g.,the second switch 110) is switched “off” (i.e. the electrons are blockedfrom the p-doped region 138). In some implementations, the first controlsignal 122 and the second control signal 132 may be voltages that aredifferential to each other.

In general, a difference between the Fermi level of a p-doped region andthe Fermi level of an n-doped region creates an electric field betweenthe two regions. In the first switch 108, an electric field is createdbetween the p-doped region 128 and the n-doped region 126. Similarly, inthe second switch 110, an electric field is created between the p-dopedregion 138 and the n-doped region 136. When the first switch 108 isswitched “on” and the second switch 110 is switched “off”, the electronsare attracted by the p-doped region 128, and the electric field betweenthe p-doped region 128 and the n-doped region 126 further carries theelectrons to the n-doped region 126. The readout circuit 124 may then beenabled to process the charges collected by the n-doped region 126. Onthe other hand, when the second switch 110 is switched “on” and thefirst switch 108 is switched “off”, the electrons are collected by thep-doped region 138, and the electric field between the p-doped region138 and the n-doped region 136 further carries the electrons to then-doped region 136. The readout circuit 134 may then be enabled toprocess the charges collected by the n-doped region 136.

In some implementations, a voltage may be applied between the p-dopedand the n-doped regions of a switch to operate the switch in anavalanche regime to increase the sensitivity of the dual-switchphotodiode 100. For example, when the distance between the p-dopedregion 128 and the n-doped region 126 is about 100 nm, it is possible toapply a voltage that is less than 7 V to create an avalanche gainbetween the p-doped region 128 and the n-doped region 126.

In some implementations, the substrate 102 may be coupled to an externalcontrol 116. For example, the substrate 102 may be coupled to ground. Insome other implementations, the substrate 102 may be floated and notcoupled to any external control.

FIG. 1B is an example dual-switch photodiode 160 for converting anoptical signal to an electrical signal. The dual-switch photodiode 160is similar to the dual-switch photodiode 100 in FIG. 1A, but that thefirst switch 108 and the second switch 110 further includes an n-wellregion 152 and an n-well region 154, respectively. In addition, theabsorption layer 106 may be a p-doped region. In some implementations,the doping level of the n-well regions 152 and 154 may range from 10¹⁵cm⁻³ to 10¹⁷ cm⁻³. The doping level of the absorption layer 106 mayrange from 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 128, the n-well region 152, thep-doped absorption layer 106, the n-well region 154, and the p-dopedregion 138 forms a PNPNP junction structure. In general, the PNPNPjunction structure reduces a conduction current from the first controlsignal 122 to the second control signal 132, or alternatively from thesecond control signal 132 to the first control signal 122. Thearrangement of the n-doped region 126, the p-doped absorption layer 106,and the n-doped region 136 forms an NPN junction structure. In general,the NPN junction structure reduces a charge coupling from the firstreadout circuit 124 to the second readout circuit 134, or alternativelyfrom the second readout circuit 134 to the first readout circuit 124.

In some implementations, the p-doped region 128 is formed entirelywithin the n-well region 152. In some other implementations, the p-dopedregion 128 is partially formed in the n-well region 152. For example, aportion of the p-doped region 128 may be formed by implanting thep-dopants in the n-well region 152, while another portion of the p-dopedregion 128 may be formed by implanting the p-dopants in the absorptionlayer 106. Similarly, in some implementations, the p-doped region 138 isformed entirely within the n-well region 154. In some otherimplementations, the p-doped region 138 is partially formed in then-well region 154.

FIG. 1C is an example dual-switch photodiode 170 for converting anoptical signal to an electrical signal. The dual-switch photodiode 170is similar to the dual-switch photodiode 100 in FIG. 1A, but that theabsorption layer 106 further includes an n-well region 156. In addition,the absorption layer 106 may be a p-doped region. In someimplementations, the doping level of the n-well region 156 may rangefrom 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. The doping level of the absorption layer106 may range from 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 128, the n-well region 156, andthe p-doped region 138 forms a PNP junction structure. In general, thePNP junction structure reduces a conduction current from the firstcontrol signal 122 to the second control signal 132, or alternativelyfrom the second control signal 132 to the first control signal 122. Thearrangement of the n-doped region 126, the p-doped absorption layer 106,and the n-doped region 136 forms an NPN junction structure. In general,the NPN junction structure reduces a charge coupling from the firstreadout circuit 124 to the second readout circuit 134, or alternativelyfrom the second readout circuit 134 to the first readout circuit 124. Insome implementations, if the depth of the n-well region 156 is deep, thearrangement of the n-doped region 126, the p-doped absorption layer 106,the n-well region 156, the p-doped absorption layer 106, and the n-dopedregion 136 forms an NPNPN junction structure, which further reduces acharge coupling from the first readout circuit 124 to the second readoutcircuit 134, or alternatively from the second readout circuit 134 to thefirst readout circuit 124.

In some implementations, the p-doped regions 128 and 138 are formedentirely within the n-well region 156. In some other implementations,the p-doped region 128 and 138 are partially formed in the n-well region156. For example, a portion of the p-doped region 128 may be formed byimplanting the p-dopants in the n-well region 156, while another portionof the p-doped region 128 may be formed by implanting the p-dopants inthe absorption layer 106.

FIG. 1D is an example dual-switch photodiode 180 for converting anoptical signal to an electrical signal. The dual-switch photodiode 180is similar to the dual-switch photodiode 100 in FIG. 1A, but that thedual-switch photodiode 150 further includes a p-well region 104 andn-well regions 142 and 144. In some implementations, the doping level ofthe n-well regions 142 and 144 may range from 10¹⁶ cm⁻³ to 10²⁰ cm⁻³.The doping level of the p-well region 104 may range from 10¹⁶ cm⁻³ to10²⁰ cm⁻³.

In some implementation, the absorption layer 106 may not completelyabsorb the incoming photons in the optical signal 112. For example, ifthe GeSi mesa does not completely absorb the incoming photons in the NIRoptical signal 112, the NIR optical signal 112 may penetrate into thesilicon substrate 102, where the silicon substrate 102 may absorb thepenetrated photons and generate photo-carriers deeply in the substratethat are slow to recombine. These slow photo-carriers negatively affectthe operation speed of the dual-switch photodiode.

To further remove the slow photo-carriers, the dual-switch photodiode150 may include connections that short the n-well regions 142 and 144with the p-well region 104. For example, the connections may be formedby a silicide process or a deposited metal pad that connects the n-wellregions 142 and 144 with the p-well region 104. The shorting between then-well regions 142 and 144 and the p-well region 104 allows thephoto-carriers generated in the substrate 102 to be recombined at theshorted node, and therefore improves the operation speed of thedual-switch photodiode. In some implementation, the p-well region 104 isused to minimize the electric field around the interfacial defectsbetween the absorptive layer 106 and the substrate 102 in order toreduce the device leakage current.

Although not shown in FIGS. 1A-1D, in some implementations, an opticalsignal may reach to the dual-switch photodiode from the backside of thesubstrate 102. One or more optical components may be fabricated on thebackside of the substrate 102 to focus, collimate, defocus, filter, orotherwise manipulate the optical signal.

Although not shown in FIGS. 1A-1D, in some other implementations, thefirst switch 108 and the second switch 110 may alternatively befabricated to collect holes instead of electrons. In this case, thep-doped region 128 and the p-doped region 138 would be replaced byn-doped regions, and the n-doped region 126 and the n-doped region 136would be replaced by p-doped regions. The n-well regions 142, 144, 152,154, and 156 would be replaced by p-well regions. The p-well region 104would be replaced by an n-well region.

Although not shown in FIGS. 1A-1D, in some implementations, theabsorption layer 106 may be bonded to a substrate after the fabricationof the dual-switch photodiode 100, 160, 170, and 180. The substrate maybe any material that allows the transmission of the optical signal 112to reach to the dual-switch photodiode. For example, the substrate maybe polymer or glass. In some implementations, one or more opticalcomponents may be fabricated on the carrier substrate to focus,collimate, defocus, filter, or otherwise manipulate the optical signal112.

Although not shown in FIGS. 1A-1D, in some implementations, thedual-switch photodiode 100, 160, 170, and 180 may be bonded (ex: metalto metal bonding, oxide to oxide bonding, hybrid bonding) to a secondsubstrate with circuits including control signals, and/or, readoutcircuits, and/or phase lock loop (PLL), and/or analog to digitalconverter (ADC). A metal layer may be deposited on top of thedual-switch photodiode that may be used as a reflector to reflect theoptical signal incident from the backside. An oxide layer may beincluded between the metal layer and the absorptive layer to increasethe reflectivity. The metal layer may also be used as the bonding layerfor the wafer-bonding process. In some implementations, one or moreswitches similar to 108 and 110 can be added for interfacing controlsignals/readout circuits.

Although not shown in FIG. 1A-1D, in some implementations, theabsorption layer 106 may be partially or fully embedded/recessed in thesubstrate 102 to relieve the surface topography and so ease thefabrication process. An example of the embedment technique is describedin U.S. patent application Ser. No. 15/228,282 titled “Germanium-SiliconLight Sensing Apparatus,” which is fully incorporated by referenceherein.

FIG. 2A is an example dual-switch photodiode 200 for converting anoptical signal to an electrical signal, where the first switch 208 andthe second switch 210 are fabricated on a substrate 202. The dual-switchphotodiode 200 includes an absorption layer 206 fabricated on asubstrate 202. The substrate 202 may be any suitable substrate wheresemiconductor devices can be fabricated on. For example, the substrate202 may be a silicon substrate.

In general, the absorption layer 206 receives an optical signal 212 andconverts the optical signal 212 into electrical signals. The absorptionlayer 206 is similar to the absorption layer 106. In someimplementations, the absorption layer 206 may include a p-doped region209. The p-doped region 209 may repel the photo-electrons from theabsorption layer 206 to the substrate 202 and thereby increase theoperation speed. For example, the p-doped region 209 may have a p+doping, where the dopant concentration is as high as a fabricationprocess may achieve, e.g., the peak concentration may be about 5×10²⁰cm⁻³ when the absorption layer 206 is germanium and doped with boron. Insome implementation, the doping concentration of the p-doped region 209may be lower than 5×10²⁰ cm⁻³ to ease the fabrication complexity at theexpense of an increased contact resistance.

A first switch 208 and a second switch 210 have been fabricated in thesubstrate 202. The first switch 208 is coupled to a first control signal222 and a first readout circuit 224. The second switch 210 is coupled toa second control signal 232 and a second readout circuit 234. Ingeneral, the first control signal 222 and the second control signal 232control whether the electrons or the holes generated by the absorbedphotons are collected by the first readout circuit 224 or the secondreadout circuit 234. The first control signal 222 is similar to thefirst control signal 122. The second control signal 232 is similar tothe second control signal 132. The first readout circuit 224 is similarto the first readout circuit 124. The second readout circuit 234 issimilar to the second readout circuit 134.

In some implementations, the first switch 208 and the second switch 210may be fabricated to collect electrons generated by the absorption layer206. In this case, the first switch 208 includes a p-doped region 228and an n-doped region 226. For example, the p-doped region 228 may havea p+ doping, where the activated dopant concentration may be as high asa fabrication process may achieve, e.g., the peak concentration may beabout 2×10²⁰ cm⁻³ when the substrate 202 is silicon and doped withboron. In some implementation, the doping concentration of the p-dopedregion 228 may be lower than 2×10²⁰ cm⁻³ to ease the fabricationcomplexity at the expense of an increased contact resistance. Then-doped region 226 may have an n+ doping, where the activated dopantconcentration may be as high as a fabrication process may achieve, e.g.,the peak concentration may be about 5×10²⁰ cm⁻³ when the substrate 202is silicon and doped with phosphorous. In some implementation, thedoping concentration of the n-doped region 226 may be lower than 5×10²⁰cm⁻³ to ease the fabrication complexity at the expense of an increasedcontact resistance. The distance between the p-doped region 228 and then-doped region 226 may be designed based on fabrication process designrules. In general, the closer the distance between the p-doped region228 and the n-doped region 226, the higher the switching efficiency ofthe generated photo-carriers. The second switch 210 includes a p-dopedregion 238 and an n-doped region 236. The p-doped region 238 is similarto the p-doped region 228, and the n-doped region 236 is similar to then-doped region 226.

In some implementations, the p-doped region 228 is coupled to the firstcontrol signal 222. The n-doped region 226 is coupled to the readoutcircuit 224. The p-doped region 238 is coupled to the second controlsignal 232. The n-doped region 236 is coupled to the readout circuit234. The first control signal 222 and the second control signal 232 areused to control the collection of electrons generated by the absorbedphotons. For example, when the absorption layer 206 absorbs photons inthe optical signal 212, electron-hole pairs are generated and drift ordiffuse into the substrate 202. When voltages are used, if the firstcontrol signal 222 is biased against the second control signal 232, anelectric field is created between the p-doped region 228 and the p-dopedregion 238, and free electrons from the absorption layer 206 drifttowards the p-doped region 228 or the p-doped region 238 depending onthe direction of the electric field. In some implementations, the firstcontrol signal 222 may be fixed at a voltage value V_(i), and the secondcontrol signal 232 may alternate between voltage values V_(i)±ΔV. Thedirection of the bias value determines the drift direction of theelectrons. Accordingly, when one switch (e.g., the first switch 208) isswitched “on” (i.e., the electrons drift towards the p-doped region228), the other switch (e.g., the second switch 210) is switched “off”(i.e., the electrons are blocked from the p-doped region 238). In someimplementations, the first control signal 222 and the second controlsignal 232 may be voltages that are differential to each other.

In the first switch 208, an electric field is created between thep-doped region 228 and the n-doped region 226. Similarly, in the secondswitch 210, an electric field is created between the p-doped region 238and the n-doped region 236. When the first switch 208 is switched “on”and the second switch 210 is switched “off”, the electrons are attractedby the p-doped region 228, and the electric field between the p-dopedregion 228 and the n-doped region 226 further carries the electrons tothe n-doped region 226. The readout circuit 224 may then be enabled toprocess the charges collected by the n-doped region 226. On the otherhand, when the second switch 210 is switched “on” and the first switch208 is switched “off”, the electrons are attracted by the p-doped region238, and the electric field between the p-doped region 238 and then-doped region 236 further carries the electrons to the n-doped region236. The readout circuit 234 may then be enabled to process the chargescollected by the n-doped region 236.

In some implementations, a voltage may be applied between the p-dopedand the n-doped regions of a switch to operate the switch in anavalanche regime to increase the sensitivity of the dual-switchphotodiode 200. For example, when the distance between the p-dopedregion 228 and the n-doped region 226 is about 100 nm, it is possible toapply a voltage that is less than 7 V to create an avalanche gainbetween the p-doped region 228 and the n-doped region 226.

In some implementations, the p-doped region 209 may be coupled to anexternal control 214. For example, the p-doped region 209 may be coupledto ground. In some implementations, the p-doped region 209 may befloated and not coupled to any external control. In someimplementations, the substrate 202 may be coupled to an external control216. For example, the substrate 202 may be coupled to ground. In someother implementations, the substrate 202 may be floated and not coupledto any external control.

FIG. 2B is an example dual-switch photodiode 250 for converting anoptical signal to an electrical signal. The dual-switch photodiode 250is similar to the dual-switch photodiode 200 in FIG. 2A, but that thefirst switch 208 and the second switch 210 further includes an n-wellregion 252 and an n-well region 254, respectively. In addition, theabsorption layer 206 may be a p-doped region and the substrate 202 maybe a p-doped substrate. In some implementations, the doping level of then-well regions 252 and 254 may range from 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. Thedoping level of the absorption layer 206 and the substrate 202 may rangefrom 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 228, the n-well region 252, thep-doped substrate 202, the n-well region 254, and the p-doped region 238forms a PNPNP junction structure. In general, the PNPNP junctionstructure reduces a conduction current from the first control signal 222to the second control signal 232, or alternatively from the secondcontrol signal 232 to the first control signal 222. The arrangement ofthe n-doped region 226, the p-doped substrate 202, and the n-dopedregion 236 forms an NPN junction structure. In general, the NPN junctionstructure reduces a charge coupling from the first readout circuit 224to the second readout circuit 234, or alternatively from the secondreadout circuit 234 to the first readout circuit 224.

In some implementations, the p-doped region 228 is formed entirelywithin the n-well region 252. In some other implementations, the p-dopedregion 228 is partially formed in the n-well region 252. For example, aportion of the p-doped region 228 may be formed by implanting thep-dopants in the n-well region 252, while another portion of the p-dopedregion 228 may be formed by implanting the p-dopants in the substrate202. Similarly, in some implementations, the p-doped region 238 isformed entirely within the n-well region 254. In some otherimplementations, the p-doped region 238 is partially formed in then-well region 254.

FIG. 2C is an example dual-switch photodiode 260 for converting anoptical signal to an electrical signal. The dual-switch photodiode 260is similar to the dual-switch photodiode 200 in FIG. 2A, but that thesubstrate 202 further includes an n-well region 244. In addition, theabsorption layer 206 may be a p-doped region and the substrate 202 maybe a p-doped substrate. In some implementations, the doping level of then-well region 244 may range from 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. The dopinglevel of the absorption layer 206 and the substrate 202 may range from10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 228, the n-well region 244, andthe p-doped region 238 forms a PNP junction structure. In general, thePNP junction structure reduces a conduction current from the firstcontrol signal 222 to the second control signal 232, or alternativelyfrom the second control signal 232 to the first control signal 222. Thearrangement of the n-doped region 226, the p-doped substrate 202, andthe n-doped region 236 forms an NPN junction structure. In general, theNPN junction structure reduces a charge coupling from the first readoutcircuit 224 to the second readout circuit 234, or alternatively from thesecond readout circuit 234 to the first readout circuit 224. In someimplementations, if the depth of the n-well 244 is deep, the arrangementof the n-doped region 226, the p-doped substrate 202, the n-well region244, the p-doped substrate 202, and the n-doped region 236 forms anNPNPN junction structure, which further reduces a charge coupling fromthe first readout circuit 224 to the second readout circuit 234, oralternatively from the second readout circuit 234 to the first readoutcircuit 224. In some implementations, the n-well region 244 alsoeffectively reduces the potential energy barrier perceived by theelectrons flowing from the absorption layer 206 to the substrate 202.

In some implementations, the p-doped regions 228 and 238 are formedentirely within the n-well region 244. In some other implementations,the p-doped regions 228 and 238 are partially formed in the n-wellregion 244. For example, a portion of the p-doped region 228 may beformed by implanting the p-dopants in the n-well region 244, whileanother portion of the p-doped region 228 may be formed by implantingthe p-dopants in the substrate 202.

FIG. 2D is an example dual-switch photodiode 270 for converting anoptical signal to an electrical signal. The dual-switch photodiode 270is similar to the dual-switch photodiode 200 in FIG. 2A, but that thedual-switch photodiode 270 further includes one or more p-well regions246 and one or more p-well regions 248. In some implementations, the oneor more p-well regions 246 and the one or more p-well regions 248 may bepart of a ring structure that surrounds the first switch 208 and thesecond switch 210. In some implementations, the doping level of the oneor more p-well regions 246 and 248 may range from 10¹⁵ cm⁻³ to 10²⁰cm⁻³. The one or more p-well regions 246 and 248 may be used as anisolation of photo-electrons from the neighboring pixels.

Although not shown in FIG. 2A-2D, in some implementations, an opticalsignal may reach to the dual-switch photodiode from the backside of thesubstrate 202. One or more optical components may be fabricated on thebackside of the substrate 202 to focus, collimate, defocus, filter, orotherwise manipulate the optical signal.

Although not shown in FIG. 2A-2D, in some other implementations, thefirst switch 208 and the second switch 210 may alternatively befabricated to collect holes instead of electrons. In this case, thep-doped region 228, the p-doped region 238, and the p-doped region 209would be replaced by n-doped regions, and the n-doped region 226 and then-doped region 236 would be replaced by p-doped regions. The n-wellregions 252, 254, and 244 would be replaced by p-well regions. Thep-well regions 246 and 248 would be replaced by n-well regions.

Although not shown in FIG. 2A-2D, in some implementations, theabsorption layer 206 may be bonded to a substrate after the fabricationof the dual-switch photodiode 200, 250, 260, and 270. The carriersubstrate may be any material that allows the transmission of theoptical signal 212 to reach to the dual-switch photodiode. For example,the substrate may be polymer or glass. In some implementations, one ormore optical components may be fabricated on the carrier substrate tofocus, collimate, defocus, filter, or otherwise manipulate the opticalsignal 212.

Although not shown in FIGS. 2A-2D, in some implementations, thedual-switch photodiode 200, 250, 260, and 270 may be bonded (ex: metalto metal bonding, oxide to oxide bonding, hybrid bonding) to a secondsubstrate with circuits including control signals, and/or, readoutcircuits, and/or phase lock loop (PLL), and/or analog to digitalconverter (ADC). A metal layer may be deposited on top of thedual-switch photodiode that may be used as a reflector to reflect theoptical signal incident from the backside. An oxide layer may beincluded between the metal layer and the absorptive layer to increasethe reflectivity. The metal layer may also be used as the bonding layerfor the wafer-bonding process. In some implementations, one or moreswitches similar to 208 and 210 can be added for interfacing controlsignals/readout circuits.

Although not shown in FIG. 2A-2D, in some implementations, theabsorption layer 206 may be partially or fully embedded/recessed in thesubstrate 202 to relieve the surface topography and so ease thefabrication process. An example of the embedment technique is describedin U.S. patent application Ser. No. 15/228,282 titled “Germanium-SiliconLight Sensing Apparatus,” which is fully incorporated by referenceherein.

FIG. 3A is an example dual-switch photodiode 300 for converting anoptical signal to an electrical signal, where first switches 308 a and308 b, and second switch 310 a and 310 b are fabricated in a verticalarrangement on a substrate 302. One characteristic with the dual-switchphotodiode 100 or the dual-switch photodiode 200 is that the larger theoptical window size d, the longer the photo-electron transit timerequired for an electron drifts or diffuses from one switch to the otherswitch. The operation speed of the photodiode may therefore be affected.The dual-switch photodiode 300 may further improve the operation speedby arranging the p-doped regions and the n-doped regions of the switchesin a vertical arrangement. Using this vertical arrangement, thephoto-electron transit distance is limited mostly by the thickness t(e.g., ˜1 μm) of the absorption layer instead of the window size d(e.g., ˜10 μm) of the absorption layer. The dual-switch photodiode 300includes an absorption layer 306 fabricated on a substrate 302. Thesubstrate 302 may be any suitable substrate where semiconductor devicescan be fabricated on. For example, the substrate 302 may be a siliconsubstrate.

In general, the absorption layer 306 receives an optical signal 312 andconverts the optical signal 312 into electrical signals. The absorptionlayer 306 is similar to the absorption layer 206. In someimplementations, the absorption layer 306 may include a p-doped region309. The p-doped region 309 is similar to the p-doped region 209.

First switches 308 a and 308 b, and second switches 310 a and 310 b havebeen fabricated in the substrate 302. Notably, although FIG. 3A onlyshows two first switches 308 a and 308 b and two second switches 310 aand 310 b, the number of first switches and second switches may be moreor less. The first switches 308 a and 308 b are coupled to a firstcontrol signal 322 and a first readout circuit 324. The second switches310 a and 310 b are coupled to a second control signal 332 and a secondreadout circuit 334.

In general, the first control signal 322 and the second control signal332 control whether the electrons or the holes generated by the absorbedphotons are collected by the first readout circuit 324 or the secondreadout circuit 334. The first control signal 322 is similar to thefirst control signal 122. The second control signal 332 is similar tothe second control signal 132. The first readout circuit 324 is similarto the first readout circuit 124. The second readout circuit 334 issimilar to the second readout circuit 134. In some implementations, thefirst switches 308 a and 308 b, and the second switches 310 a and 310 bmay be fabricated to collect electrons generated by the absorption layer306. In this case, the first switches 308 a and 308 b include p-dopedregions 328 a and 328 b, and n-doped regions 326 a and 326 b,respectively. For example, the p-doped regions 328 a and 328 b may havea p+ doping, where the activated dopant concentration may be as high asa fabrication process may achieve, e.g., the peak concentration may beabout 2×10²⁰ cm⁻³ when the substrate 302 is silicon and doped withboron. In some implementation, the doping concentration of the p-dopedregions 328 a and 328 b may be lower than 2×10²⁰ cm⁻³ to ease thefabrication complexity at the expense of an increased contactresistance. The n-doped regions 326 a and 326 b may have an n+ doping,where the activated dopant concentration may be as high as a fabricationprocess may achieve, e.g., the peak concentration may be about 5×10²⁰cm⁻³ when the substrate 302 is silicon and doped with phosphorous. Insome implementation, the doping concentration of the n-doped regions 326a and 326 b may be lower than 5×10²⁰ cm⁻³ to ease the fabricationcomplexity at the expense of an increased contact resistance. Thedistance between the p-doped region 328 a and the n-doped region 326 amay be designed based on fabrication process design rules. For example,the distance between the p-doped region 328 a and the n-doped region 326a may be controlled by the energies associated with the dopant implants.In general, the closer the distance between the p-doped regions 328a/328 b and the n-doped regions 326 a/326 b, the higher the switchingefficiency of the generated photo-carriers. The second switches 310 aand 310 b includes p-doped regions 338 a and 338 b, and n-doped regions336 a and 336 b, respectively. The p-doped regions 338 a/338 b aresimilar to the p-doped regions 328 a/328 b, and the n-doped regions 336a/336 b are similar to the n-doped region 326 a/326 b.

In some implementations, the p-doped regions 328 a and 328 b are coupledto the first control signal 322. The n-doped regions 326 a and 326 b arecoupled to the readout circuit 324. The p-doped regions 338 a and 338 bare coupled to the second control signal 332. The n-doped regions 336 aand 336 b are coupled to the readout circuit 334. The first controlsignal 322 and the second control signal 332 are used to control thecollection of electrons generated by the absorbed photons. For example,when the absorption layer 306 absorbs photons in the optical signal 312,electron-hole pairs are generated and drift or diffuse into thesubstrate 302. When voltages are used, if the first control signal 322is biased against the second control signal 332, electric fields arecreated between the p-doped region 309 and the p-doped regions 328 a/328b or the p-doped regions 338 a/338 b, and free electrons from theabsorption layer 306 drift towards the p-doped regions 328 a/328 b orthe p-doped regions 338 a/338 b depending on the directions of theelectric fields. In some implementations, the first control signal 322may be fixed at a voltage value V_(i), and the second control signal 332may alternate between voltage values V_(i)±ΔV. The direction of the biasvalue determines the drift direction of the electrons. Accordingly, whenone group of switches (e.g., first switches 308 a and 308 b) areswitched “on” (i.e., the electrons drift towards the p-doped regions 328a and 328 b), the other group of switches (e.g., the second switches 310a and 310 b) are switched “off” (i.e., the electrons are blocked fromthe p-doped regions 338 a and 338 b). In some implementations, the firstcontrol signal 322 and the second control signal 332 may be voltagesthat are differential to each other.

In each of the first switches 308 a/308 b, an electric field is createdbetween the p-doped region 328 a/328 b and the n-doped region 326 a/326b. Similarly, in each of the second switches 310 a/310 b, an electricfield is created between the p-doped region 338 a/338 b and the n-dopedregion 336 a/336 b. When the first switches 308 a and 308 b are switched“on” and the second switches 310 a and 310 b are switched “off”, theelectrons are attracted by the p-doped regions 328 a and 328 b, and theelectric field between the p-doped region 328 a and the n-doped region326 a further carries the electrons to the n-doped region 326 a.Similarly, the electric field between the p-doped region 328 b and then-doped region 326 b further carries the electrons to the n-doped region326 b. The readout circuit 324 may then be enabled to process thecharges collected by the n-doped regions 326 a and 326 b. On the otherhand, when the second switches 310 a and 310 b are switched “on” and thefirst switches 308 a and 308 b are switched “off”, the electrons areattracted by the p-doped regions 338 a and 338 b, and the electric fieldbetween the p-doped region 338 a and the n-doped region 336 a furthercarries the electrons to the n-doped region 336 a. Similarly, theelectric field between the p-doped region 338 b and the n-doped region336 b further carries the electrons to the n-doped region 336 b. Thereadout circuit 334 may then be enabled to process the amount of chargescollected by the n-doped regions 336 a and 336 b.

In some implementations, a voltage may be applied between the p-dopedand the n-doped regions of a switch to operate the switch in anavalanche regime to increase the sensitivity of the dual-switchphotodiode 300. For example, when the distance between the p-dopedregion 328 a and the n-doped region 326 a is about 100 nm, it ispossible to apply a voltage that is less than 7 V to create an avalanchegain between the p-doped region 328 a and the n-doped region 326 a.

In some implementations, the p-doped region 309 may be coupled to anexternal control 314. For example, the p-doped region 309 may be coupledto ground. In some implementations, the p-doped region 309 may befloated and not coupled to any external control. In someimplementations, the substrate 302 may be coupled to an external control316. For example, the substrate 302 may be coupled to ground. In someother implementations, the substrate 302 may be floated and not coupledto any external control.

FIG. 3B is an example dual-switch photodiode 360 for converting anoptical signal to an electrical signal. The dual-switch photodiode 360is similar to the dual-switch photodiode 300 in FIG. 3A, but that thedual-switch photodiode 360 further includes an n-well region 344. Inaddition, the absorption layer 360 may be a p-doped region and thesubstrate 302 may be a p-doped substrate. In some implementations, thedoping level of the n-well region 344 may range from 10¹⁵ cm⁻³ to 10¹⁷cm⁻³. The doping level of the absorption layer 360 and the substrate 302may range from 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 328 a, the n-well region 344, andthe p-doped region 338 a forms a PNP junction structure. Similarly, thearrangement of the p-doped region 328 b, the n-well region 344, and thep-doped region 338 b forms another PNP junction structure. In general,the PNP junction structure reduces a conduction current from the firstcontrol signal 322 to the second control signal 332, or alternativelyfrom the second control signal 332 to the first control signal 322. Thearrangement of the n-doped region 326 a, the p-doped substrate 302, andthe n-doped region 336 a forms an NPN junction structure. Similarly, thearrangement of the n-doped region 326 b, the p-doped substrate 302, andthe n-doped region 336 b forms an NPN junction structure. In general,the NPN junction structure reduces a charge coupling from the firstreadout circuit 324 to the second readout circuit 334, or alternativelyfrom the second readout circuit 334 to the first readout circuit 324. Insome implementations, the n-well region 344 also effectively reduces thepotential energy barrier perceived by the electrons flowing from theabsorption layer 306 to the substrate 302.

In some implementations, the p-doped regions 328 a, 338 a, 328 b, and338 b are formed entirely within the n-well region 344. In some otherimplementations, the p-doped regions 328 a, 338 a, 328 b, and 338 b arepartially formed in the n-well region 344. For example, a portion of thep-doped region 328 a may be formed by implanting the p-dopants in then-well region 344, while another portion of the p-doped region 328A maybe formed by implanting the p-dopants in the substrate 302.

FIG. 3C is an example dual-switch photodiode 370 for converting anoptical signal to an electrical signal. The dual-switch photodiode 370is similar to the dual-switch photodiode 300 in FIG. 3A, but that thedual-switch photodiode 370 further includes one or more p-well regions346 and one or more p-well regions 348. In some implementations, the oneor more p well regions 346 and the one or more p-well regions 348 may bepart of a ring structure that surrounds the first switches 308 a and 308b, and the second switches 310 a and 310 b. In some implementations, thedoping level of the one or more p-well regions may range from 10¹⁵ cm⁻³to 10²⁰ cm⁻³. The one or more p-well regions 346 and 348 may be used asan isolation of photo-electrons from the neighboring pixels.

FIG. 3D shows cross-sectional views of the example dual-switchphotodiode 380. FIG. 3D shows that the p-doped regions 328 a and 328 bof the first switches 308 a and 308 b, and the p-doped regions 338 a and338 b of the second switches 310 a and 310 b may be arranged on a firstplane 362 of the substrate 302 in an interdigitated arrangement. FIG. 3Dfurther shows that the n-doped regions 326 a and 326 b of the firstswitches 308 a and 308 b, and the n-doped regions 336 a and 336 b of thesecond switches 310 a and 310 b may be arranged on a second plane 364 ofthe substrate 302 in an interdigitated arrangement.

Although not shown in FIG. 3A-3D, in some implementations, an opticalsignal may reach to the dual-switch photodiode from the backside of thesubstrate 302. One or more optical components may be fabricated on thebackside of the substrate 302 to focus, collimate, defocus, filter, orotherwise manipulate the optical signal.

Although not shown in FIG. 3A-3D, in some other implementations, thefirst switches 308 a and 308 b, and the second switches 310 a and 310 bmay alternatively be fabricated to collect holes instead of electrons.In this case, the p-doped regions 328 a and 328 b, the p-doped regions338 a and 338 b, and the p-doped region 309 would be replaced by n-dopedregions, and the n-doped regions 326 a and 326 b, and the n-dopedregions 336 a and 336 b would be replaced by p-doped regions. The n-wellregion 344 would be replaced by a p-well region. The p-well regions 346and 348 would be replaced by n-well regions.

Although not shown in FIG. 3A-3D, in some implementations, theabsorption layer 306 may be bonded to a substrate after the fabricationof the dual-switch photodiode 300, 360, 370, and 380. The substrate maybe any material that allows the transmission of the optical signal 312to reach to the dual-switch photodiode. For example, the substrate maybe polymer or glass. In some implementations, one or more opticalcomponents may be fabricated on the carrier substrate to focus,collimate, defocus, filter, or otherwise manipulate the optical signal312.

Although not shown in FIGS. 3A-3D, in some implementations, thedual-switch photodiode 300, 360, 370, and 380 may be bonded (ex: metalto metal bonding, oxide to oxide bonding, hybrid bonding) to a secondsubstrate with circuits including control signals, and/or, readoutcircuits, and/or phase lock loop (PLL), and/or analog to digitalconverter (ADC). A metal layer may be deposited on top of thedual-switch photodiode that may be used as a reflector to reflect theoptical signal incident from the backside. An oxide layer may beincluded between the metal layer and the absorptive layer to increasethe reflectivity. The metal layer may also be used as the bonding layerfor the wafer-bonding process. In some implementations, one or moreswitches similar to 308 a (or 308 b) and 310 a (or 310 b) can be addedfor interfacing control signals/readout circuits.

Although not shown in FIG. 3A-3D, in some implementations, theabsorption layer 306 may be partially or fully embedded/recessed in thesubstrate 302 to relieve the surface topography and so ease thefabrication process. An example of the embedment technique is describedin U.S. patent application Ser. No. 15/228,282 titled “Germanium-SiliconLight Sensing Apparatus,” which is fully incorporated by referenceherein.

FIG. 4A is an example dual-switch photodiode 400 for converting anoptical signal to an electrical signal. The dual-switch photodiode 400includes an absorption layer 406 fabricated on a substrate 402. Thesubstrate 402 may be any suitable substrate where semiconductor devicescan be fabricated on. For example, the substrate 402 may be a siliconsubstrate. The absorption layer 406 includes a first switch 408 and asecond switch 410.

In general, the absorption layer 406 receives an optical signal 412 andconverts the optical signal 412 into electrical signals. The absorptionlayer 406 is selected to have a high absorption coefficient at thedesired wavelength range. For NIR wavelengths, the absorption layer 406may be a GeSi mesa, where the GeSi absorbs photons in the optical signal412 and generates electron-hole pairs. The material composition ofgermanium and silicon in the GeSi mesa may be selected for specificprocesses or applications. In some implementations, the absorption layer406 is designed to have a thickness t. For example, for 850 nmwavelength, the thickness of the GeSi mesa may be approximately 1 μm tohave a substantial quantum efficiency. In some implementations, thesurface of the absorption layer 406 is designed to have a specificshape. For example, the GeSi mesa may be circular, square, orrectangular depending on the spatial profile of the optical signal 412on the surface of the GeSi mesa. In some implementations, the absorptionlayer 406 is designed to have a lateral dimension d for receiving theoptical signal 412. For example, the GeSi mesa may have a circularshape, where d can range from 1 μm to 50 μm.

A first switch 408 and a second switch 410 have been fabricated in theabsorption layer 406 and the substrate 402. The first switch 408 iscoupled to a first control signal 422 and a first readout circuit 424.The second switch 410 is coupled to a second control signal 432 and asecond readout circuit 434. In general, the first control signal 422 andthe second control signal 432 control whether the electrons or the holesgenerated by the absorbed photons are collected by the first readoutcircuit 424 or the second readout circuit 434.

In some implementations, the first switch 408 and the second switch 410may be fabricated to collect electrons. In this case, the first switch408 includes a p-doped region 428 implanted in the absorption layer 406and an n-doped region 426 implanted in the substrate 402. For example,the p-doped region 428 may have a p+ doping, where the activated dopantconcentration may be as high as a fabrication process may achieve, e.g.,the peak concentration may be about 5×10²⁰ cm⁻³ when the absorptionlayer 106 is germanium and doped with boron. In some implementation, thedoping concentration of the p-doped region 428 may be lower than 5×10²⁰cm⁻³ to ease the fabrication complexity at the expense of an increasedcontact resistance. The n-doped region 426 may have an n+ doping, wherethe activated dopant concentration may be as high as a fabricationprocess may achieve, e.g., e.g., the peak concentration may be about5×10²⁰ cm⁻³ when the substrate 402 is silicon and doped withphosphorous. In some implementation, the doping concentration of then-doped region 426 may be lower than 5×10²⁰ cm⁻³ to ease the fabricationcomplexity at the expense of an increased contact resistance. Thedistance between the p-doped region 428 and the n-doped region 426 maybe designed based on fabrication process design rules. In general, thecloser the distance between the p-doped region 428 and the n-dopedregion 426, the higher the switching efficiency of the generatedphoto-carriers. The second switch 410 includes a p-doped region 438 andan n-doped region 436. The p-doped region 438 is similar to the p-dopedregion 428, and the n-doped region 436 is similar to the n-doped region426.

In some implementations, the p-doped region 428 is coupled to the firstcontrol signal 422. For example, the p-doped region 428 may be coupledto a voltage source, where the first control signal 422 may be an ACvoltage signal from the voltage source. In some implementations, then-doped region 426 is coupled to the readout circuit 424. The readoutcircuit 424 may be in a three-transistor configuration consisting of areset gate, a source-follower, and a selection gate, or any suitablecircuitry for processing charges. In some implementations, the readoutcircuit 424 may be fabricated on the substrate 402. In some otherimplementations, the readout circuit 424 may be fabricated on anothersubstrate and integrated/co-packaged with the dual-switch photodiode 400via die/wafer bonding or stacking.

The p-doped region 438 is coupled to the second control signal 432. Forexample, the p-doped region 438 may be coupled to a voltage source,where the second control signal 432 may be an AC voltage signal havingan opposite phase from the first control signal 422. In someimplementations, the n-doped region 436 is coupled to the readoutcircuit 434. The readout circuit 434 may be similar to the readoutcircuit 424.

The first control signal 422 and the second control signal 432 are usedto control the collection of electrons generated by the absorbedphotons. For example, when voltages are used, if the first controlsignal 422 is biased against the second control signal 432, an electricfield is created between the p-doped region 428 and the p-doped region438, and free electrons drift towards the p-doped region 428 or thep-doped region 438 depending on the direction of the electric field. Insome implementations, the first control signal 422 may be fixed at avoltage value V_(i), and the second control signal 432 may alternatebetween voltage values V_(i)±ΔV. The direction of the bias valuedetermines the drift direction of the electrons. Accordingly, when oneswitch (e.g., the first switch 408) is switched “on” (i.e., theelectrons drift towards the p-doped region 428), the other switch (e.g.,the second switch 410) is switched “off” (i.e. the electrons are blockedfrom the p-doped region 438). In some implementations, the first controlsignal 422 and the second control signal 432 may be voltages that aredifferential to each other.

In general, a difference between the Fermi level of a p-doped region andthe Fermi level of an n-doped region creates an electric field betweenthe two regions. In the first switch 408, an electric field is createdbetween the p-doped region 428 and the n-doped region 426. Similarly, inthe second switch 410, an electric field is created between the p-dopedregion 438 and the n-doped region 436. When the first switch 408 isswitched “on” and the second switch 410 is switched “off”, the electronsare attracted by the p-doped region 428, and the electric field betweenthe p-doped region 428 and the n-doped region 426 further carries theelectrons to the n-doped region 426. The readout circuit 424 may then beenabled to process the charges collected by the n-doped region 426. Onthe other hand, when the second switch 410 is switched “on” and thefirst switch 408 is switched “off”, the electrons are attracted by thep-doped region 438, and the electric field between the p-doped region438 and the n-doped region 436 further carries the electrons to then-doped region 436. The readout circuit 434 may then be enabled toprocess the charges collected by the n-doped region 436.

In some implementations, the substrate 402 may be coupled to an externalcontrol 416. For example, the substrate 402 may be coupled to ground. Insome other implementations, the substrate 402 may be floated and notcoupled to any external control.

FIG. 4B is an example dual-switch photodiode 450 for converting anoptical signal to an electrical signal. The dual-switch photodiode 450is similar to the dual-switch photodiode 400 in FIG. 4A, but that thefirst switch 408 and the second switch 410 further includes an n-wellregion 452 and an n-well region 454, respectively. In addition, theabsorption layer 406 may be a p-doped layer and the substrate 402 may bea p-doped substrate. In some implementations, the doping level of then-well regions 452 and 454 may range from 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. Thedoping level of the absorption layer 406 and the substrate 402 may rangefrom 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 428, the n-well region 452, theabsorption layer 406, the n-well region 454, and the p-doped region 438forms a PNPNP junction structure. In general, the PNPNP junctionstructure reduces a conduction current from the first control signal 422to the second control signal 432, or alternatively from the secondcontrol signal 432 to the first control signal 422.

The arrangement of the n-doped region 426, the p-doped substrate 402,and the n-doped region 436 forms an NPN junction structure. In general,the NPN junction structure reduces a charge coupling from the firstreadout circuit 424 to the second readout circuit 434, or alternativelyfrom the second readout circuit 434 to the first readout circuit 424.

In some implementations, the p-doped region 428 is formed entirelywithin the n-well region 452. In some other implementations, the p-dopedregion 428 is partially formed in the n-well region 452. For example, aportion of the p-doped region 428 may be formed by implanting thep-dopants in the n-well region 452, while another portion of the p-dopedregion 428 may be formed by implanting the p-dopants in the absorptionlayer 406. Similarly, in some implementations, the p-doped region 438 isformed entirely within the n-well region 454. In some otherimplementations, the p-doped region 438 is partially formed in then-well region 454.

FIG. 4C is an example dual-switch photodiode 460 for converting anoptical signal to an electrical signal. The dual-switch photodiode 460is similar to the dual-switch photodiode 400 in FIG. 4A, but that theabsorption layer 406 further includes an n-well region 456. In addition,the absorption layer 406 may be a p-doped region and the substrate 402may be a p-doped substrate. In some implementations, the doping level ofthe n-well region 456 may range from 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. The dopinglevel of the absorption layer 406 and the substrate 402 may range from10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 428, the n-well region 456, andthe p-doped region 438 forms a PNP junction structure. In general, thePNP junction structure reduces a conduction current from the firstcontrol signal 422 to the second control signal 432, or alternativelyfrom the second control signal 432 to the first control signal 422.

The arrangement of the n-doped region 426, the p-doped absorption layer406, and the n-doped region 436 forms an NPN junction structure. Ingeneral, the NPN junction structure reduces a charge coupling from thefirst readout circuit 424 to the second readout circuit 434, oralternatively from the second readout circuit 434 to the first readoutcircuit 424.

In some implementations, the p-doped regions 428 and 438 are formedentirely within the n-well region 456. In some other implementations,the p-doped region 428 and 438 are partially formed in the n-well region456. For example, a portion of the p-doped region 428 may be formed byimplanting the p-dopants in the n-well region 456, while another portionof the p-doped region 428 may be formed by implanting the p-dopants inthe absorption layer 406.

FIG. 4D is an example dual-switch photodiode 470 for converting anoptical signal to an electrical signal. The dual-switch photodiode 470is similar to the dual-switch photodiode 460 in FIG. 4C, but that then-well region 458 is formed to extend from the absorption layer 406 intothe substrate 202. In addition, the absorption layer 406 may be ap-doped region and the substrate 402 may be a p-doped substrate. In someimplementations, the doping level of the n-well region 456 may rangefrom 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. The doping level of the absorption layer406 and the substrate 402 may range from 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 428, the n-well region 458, andthe p-doped region 438 forms a PNP junction structure, which furtherreduces a conduction current from the first control signal 422 to thesecond control signal 432, or alternatively from the second controlsignal 432 to the first control signal 422. The arrangement of then-doped region 426, the p-doped substrate 402, the n-well region 458,the p-doped substrate 402, and the n-doped region 436 forms an NPNPNjunction structure, which further reduces a charge coupling from thefirst readout circuit 424 to the second readout circuit 434, oralternatively from the second readout circuit 434 to the first readoutcircuit 424. In some implementations, the n-well region 458 alsoeffectively reduces the potential energy barrier perceived by theelectrons flowing from the absorption layer 406 to the substrate 402.

FIG. 4E is an example dual-switch photodiode 480 for converting anoptical signal to an electrical signal. The dual-switch photodiode 480is similar to the dual-switch photodiode 400 in FIG. 4A, but that thedual-switch photodiode 480 further includes one or more p-well regions446 and one or more p-well regions 448. In some implementations, the oneor more p-well regions 446 and the one or more p-well regions 448 may bepart of a ring structure that surrounds the first switch 408 and thesecond switch 410. In some implementations, the doping level of the oneor more p-well regions 446 and 448 may range from 10¹⁵ cm⁻³ to 10²⁰cm⁻³. The one or more p-well regions 446 and 448 may be used as anisolation of photo-electrons from the neighboring pixels.

Although not shown in FIG. 4A-4E, in some implementations, an opticalsignal may reach to the dual-switch photodiode from the backside of thesubstrate 402. One or more optical components may be fabricated on thebackside of the substrate 402 to focus, collimate, defocus, filter, orotherwise manipulate the optical signal.

Although not shown in FIG. 4A-4E, in some other implementations, thefirst switch 408 and the second switch 410 may alternatively befabricated to collect holes instead of electrons. In this case, thep-doped region 428 and the p-doped region 438 would be replaced byn-doped regions, and the n-doped region 426 and the n-doped region 436would be replaced by p-doped regions. The n-well regions 452, 454, 456,and 458 would be replaced by p-well regions. The p-well regions 446 and448 would be replaced by n-well regions.

Although not shown in FIG. 4A-4E, in some implementations, theabsorption layer 406 may be bonded to a substrate after the fabricationof the dual-switch photodiode 400, 450, 460, 470, and 480. The substratemay be any material that allows the transmission of the optical signal412 to reach to the dual-switch photodiode. For example, the substratemay be polymer or glass. In some implementations, one or more opticalcomponents may be fabricated on the carrier substrate to focus,collimate, defocus, filter, or otherwise manipulate the optical signal412.

Although not shown in FIGS. 4A-4E, in some implementations, thedual-switch photodiode 400, 450, 460, 470, and 480 may be bonded (ex:metal to metal bonding, oxide to oxide bonding, hybrid bonding) to asecond substrate with circuits including control signals, and/or,readout circuits, and/or phase lock loop (PLL), and/or analog to digitalconverter (ADC). A metal layer may be deposited on top of thedual-switch photodiode that may be used as a reflector to reflect theoptical signal incident from the backside. An oxide layer may beincluded between the metal layer and the absorptive layer to increasethe reflectivity. The metal layer may also be used as the bonding layerfor the wafer-bonding process. In some implementations, one or moreswitches similar to 408 and 410 can be added for interfacing controlsignals/readout circuits.

Although not shown in FIG. 4A-4E, in some implementations, theabsorption layer 406 may be partially or fully embedded/recessed in thesubstrate 402 to relieve the surface topography and so ease thefabrication process. An example of the embedment technique is describedin U.S. patent application Ser. No. 15/228,282 titled “Germanium-SiliconLight Sensing Apparatus,” which is fully incorporated by referenceherein.

FIG. 5A shows an example imaging system 500 for determiningcharacteristics of a target object 510. The target object 510 may be athree-dimensional object. The imaging system 500 may include atransmitter unit 502, a receiver unit 504, and a processing unit 506. Ingeneral, the transmitter unit 502 emits light 512 towards the targetobject 510. The transmitter unit 502 may include one or more lightsources, control circuitry, and/or optical elements. For example, thetransmitter unit 502 may include one or more NIR LEDs or lasers, wherethe emitted light 512 may be collimated by a collimating lens topropagate in free space.

In general, the receiver unit 504 receives the reflected light 514 thatis reflected from the target object 510. The receiver unit 504 mayinclude one or more photodiodes, control circuitry, and/or opticalelements. For example, the receiver unit 504 may include an imagesensor, where the image sensor includes multiple pixels fabricated on asemiconductor substrate. Each pixel may include one or more dual-switchphotodiodes for detecting the reflected light 514, where the reflectedlight 514 may be focused to the dual-switch photodiodes. Eachdual-switch photodiode may be a dual-switch photodiode disclosed in thisapplication.

In general, the processing unit 506 processes the photo-carriersgenerated by the receiver unit 504 and determines characteristics of thetarget object 510. The processing unit 506 may include controlcircuitry, one or more processors, and/or computer storage medium thatmay store instructions for determining the characteristics of the targetobject 510. For example, the processing unit 506 may include readoutcircuits and processors that can process information associated with thecollected photo-carriers to determine the characteristics of the targetobject 510. In some implementations, the characteristics of the targetobject 510 may be depth information of the target object 510. In someimplementations, the characteristics of the target object 510 may bematerial compositions of the target object 510.

FIG. 5B shows one example technique for determining characteristics ofthe target object 510. The transmitter unit 502 may emit light pulses512 modulated at a frequency f_(m) with a duty cycle of 50%. Thereceiver unit 504 may receive reflected light pulses 514 having a phaseshift of Φ. The dual-switch photodiodes are controlled such that thereadout circuit 1 reads the collected charges Q₁ in a phase synchronizedwith the emitted light pulses, and the readout circuit 2 reads thecollected charges Q₂ in an opposite phase with the emitted light pulses.In some implementations, the distance, D, between the imaging system 500and the target object 510 may be derived using the equation

$\begin{matrix}{{D = {\frac{c}{4f_{m}}\frac{Q_{2}}{Q_{1} + Q_{2}}}},} & (1)\end{matrix}$where c is the speed of light.

FIG. 5C shows another example technique for determining characteristicsof the target object 510. The transmitter unit 502 may emit light pulses512 modulated at a frequency f_(m) with a duty cycle of less than 50%.By reducing the duty cycle of the optical pulses by a factor of N, butincreasing the intensity of the optical pulses by a factor of N at thesame time, the signal-to-noise ratio of the received reflected lightpulses 514 may be improved while maintaining substantially the samepower consumption for the imaging system 500. This is made possible whenthe device bandwidth is increased so that the duty cycle of the opticalpulses can be decreased without distorting the pulse shape. The receiverunit 504 may receive reflected light pulses 514 having a phase shift ofΦ. The multi-gate photodiodes are controlled such that a readout circuit1 reads the collected charges Q₁′ in a phase synchronized with theemitted light pulses, and a readout circuit 2 reads the collectedcharges Q₂′ in a delayed phase with the emitted light pulses. In someimplementations, the distance, D, between the imaging system 500 and thetarget object 510 may be derived using the equation

$\begin{matrix}{D = {\frac{c}{4{Nf}_{m}}{\frac{Q_{2}^{\prime}}{Q_{1}^{\prime} + Q_{2}^{\prime}}.}}} & (2)\end{matrix}$

FIG. 6 shows an example of a flow diagram 600 for determiningcharacteristics of an object using an imaging system. The process 600may be performed by a system such as the imaging system 500.

The system receives reflected light (602). For example, the transmitterunit 502 may emit NIR light pulses 512 towards the target object 510.The receiver unit 504 may receive the reflected NIR light pulses 514that is reflected from the target object 510.

The system determines phase information (604). For example, the receiverunit 504 may include an image sensor, where the image sensor includesmultiple pixels fabricated on a semiconductor substrate. Each pixel mayinclude one or more dual-switch photodiodes for detecting the reflectedlight pulses 514. The type of dual-switch photodiodes may be adual-switch photodiode disclosed in this application, where the phaseinformation may be determined using techniques described in reference toFIG. 5B or FIG. 5C.

The system determines object characteristics (606). For example, theprocessing unit 506 may determine depth information of the object 510based on the phase information using techniques described in referenceto FIG. 5B or FIG. 5C.

In some implementations, an image sensor includes multiple pixels arefabricated on a semiconductor substrate, where each pixel may includeone or more dual-switch photodiodes 100, 160, 170, 180, 200, 250, 260,270, 300, 360, 370, 380, 400, 450, 460, 470, and 480 for detecting thereflected light as illustrated in FIGS. 5A and 5B. The isolation betweenthese pixels may be implemented based on an insulator isolation such asusing an oxide or nitride layer, or based on an implant isolation suchas using p+ or n+ region to block signal electrons or holes, or based onan intrinsic built-in energy barrier such as a using thegermanium-silicon heterojunction interface.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, various formsof the flows shown above may be used, with steps re-ordered, added, orremoved.

Various implementations may have been discussed using two-dimensionalcross-sections for easy description and illustration purpose.Nevertheless, the three-dimensional variations and derivations shouldalso be included within the scope of the disclosure as long as there arecorresponding two-dimensional cross-sections in the three-dimensionalstructures.

While this specification contains many specifics, these should not beconstrued as limitations, but rather as descriptions of featuresspecific to particular embodiments. Certain features that are describedin this specification in the context of separate embodiments may also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment mayalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination may in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems maygenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments have been described. Other embodiments arewithin the scope of the following claims. For example, the actionsrecited in the claims may be performed in a different order and stillachieve desirable results.

What is claimed is:
 1. An optical sensor comprising: a siliconsubstrate; an absorption layer supported by the silicon substrate andconfigured to absorb photons and to generate electrons and holes inresponse to the absorbed photons, wherein the absorption layer comprisesgermanium and includes a first p-doped region; a first n-doped regionformed in the silicon substrate, wherein the first n-doped region isconfigured to collect a portion of the electrons generated in theabsorption layer; and a second n-doped region formed in the siliconsubstrate, wherein the second n-doped region is in direct contact withthe absorption layer, and wherein the second n-doped region is arrangedto guide the portion of the electrons from the absorption layer to thefirst n-doped region.
 2. The optical sensor of claim 1, wherein theabsorption layer is formed on the silicon substrate as a mesa.
 3. Theoptical sensor of claim 1, wherein the absorption layer is at leastpartially embedded in the silicon substrate.
 4. The optical sensor ofclaim 1, wherein a dopant level of the second n-doped region ranges from10¹⁵ cm⁻³ to 10¹⁷ cm⁻³, and wherein a dopant level of the first n-dopedregion is higher than the dopant level of the second n-doped region. 5.The optical sensor of claim 1, further comprising an optical componentover a backside of the silicon substrate, the optical component beingconfigured to guide an optical signal through the silicon substrate tothe absorption layer.
 6. The optical sensor of claim 5, wherein theoptical signal has a wavelength that is greater than 850 nm.
 7. Theoptical sensor of claim 1, further comprising a third n-doped regionformed in the silicon substrate, wherein the third n-doped region isconfigured to collect a second portion of the electrons generated in theabsorption layer.
 8. The optical sensor of claim 1, further comprising asecond p-doped region formed in the silicon substrate and arrangedbetween the absorption layer and the first n-doped region, wherein thesecond p-doped region is coupled to a control signal and is configuredto control a flow of the portion of the electrons from the absorptionlayer to the first n-doped region.
 9. The optical sensor of claim 1,further comprising a second p-doped region formed in the absorptionlayer, wherein the second p-doped region is configured to collect aportion of the holes generated in the absorption layer.
 10. The opticalsensor of claim 1, wherein the first n-doped region is coupled to areadout circuitry configured to process the portion of the electronscollected in the first n-doped region.
 11. The optical sensor of claim10, further comprising a second silicon substrate bonded to the siliconsubstrate, wherein the readout circuitry is formed in the second siliconsubstrate.
 12. The optical sensor of claim 10, wherein the opticalsensor is configured to determine one or more characteristics of atarget object, and wherein the one or more characteristics comprise adepth of the target object or a material of the target object.
 13. Theoptical sensor of claim 1, comprising an array of pixels, wherein theabsorption layer is formed in one pixel of the array of pixels.
 14. Anoptical system comprising: a transmitter unit comprising multiple lightsources; and a receiver unit comprising an optical sensor, the opticalsensor comprising: a silicon substrate; an absorption layer supported bythe silicon substrate and configured to absorb photons and to generateelectrons and holes in response to the absorbed photons, wherein theabsorption layer comprises germanium and includes a first p-dopedregion; a first n-doped region formed in the silicon substrate, whereinthe first n-doped region is configured to collect a portion of theelectrons generated in the absorption layer; and a second n-doped regionformed in the silicon substrate, wherein the second n-doped region is indirect contact with the absorption layer, and wherein the second n-dopedregion is arranged to guide the portion of the electrons from theabsorption layer to the first n-doped region.
 15. The optical system ofclaim 14, further comprising a processing unit configured to determineone or more characteristics of a target object.
 16. The optical systemof claim 15, wherein the one or more characteristics comprise a depth ofthe target object or a material of the target object.
 17. The opticalsystem of claim 14, wherein at least one light source of the multiplelight sources has a wavelength in a near infrared wavelength range. 18.The optical system of claim 14, wherein the multiple light sourcescomprise LEDs.
 19. The optical system of claim 14, further comprising areadout circuitry coupled to the first n-doped region of the receiverunit.
 20. The optical system of claim 19, wherein the readout circuitryis formed on a second silicon substrate bonded to the silicon substrate.